1. Field of the Invention
The present invention relates to a method and system for controlling the process of etching a structure on a substrate, for example, in semiconductor manufacturing. More particularly, it relates to a system and method for control of an etch process for forming of a structure with a feature, or multiple features in the horizontal direction, for example a stepped structure, on the substrate.
2. Description of Related Art
Plasma etch processes are commonly used in conjunction with photolithography in the process of manufacturing semiconductor devices, liquid crystal displays (LCDs), light-emitting diodes (LEDs), and some photovoltaics (PVs). Generally, a layer of radiation-sensitive material, such as photoresist, is first coated on a substrate and exposed to patterned light to impart a latent image thereto. Thereafter, the exposed radiation-sensitive material is developed to remove exposed radiation-sensitive material (or unexposed, if negative tone photoresist is used), leaving a pattern of radiation-sensitive material which exposes areas to be subsequently etched, and covers areas where no etching is desired. During the etch process, for example a plasma etch process, the substrate and radiation-sensitive material pattern are exposed to energetic ions in a plasma processing chamber, so as to effect removal of the material underlying the radiation-sensitive material in order to form etched features, such as vias, trenches, etc. Following etching of the features in the underlying material, the remainder of the radiation-sensitive material is removed from the substrate using an ashing or stripping process, to expose formed etched structures ready for further processing.
In many types of devices, such as semiconductor devices, the plasma etch process is performed in a first material layer overlying a second material layer, and it is important that the etch process be stopped accurately once the etch process has formed an opening or pattern in the first material layer, without continuing to etch the underlying second material layer. In other devices, the vertical dimensions of device features, such as sidewalls, depths of vias and trenches, etc., are critical parameters regardless of whether there is an underlying layer present. In both cases, the duration of the etch process needs to be controlled accurately so as to either achieve a precise etch stop at the top of an underlying material layer, or to achieve an exact vertical dimension of etched features.
For purposes of controlling the etch process various types of endpoint control are utilized, some of which rely on analyzing the chemistry of the gas in the plasma processing chamber in order to deduce whether the etch process has progressed, for example, to an underlying material layer of a different chemical composition than the material of the layer being etched. Alternatively, in-situ metrology devices can be used to directly measure the etched features during the etch process and provide feedback control for accurately stopping the etch process once an underlying material layer has been reached, or a certain vertical feature dimension has been attained.
In modern semiconductor devices, particularly FLASH RAM devices, novel structures are increasingly frequently being used that involve stacked stepped layers, stepped pyramidal structures, or staircase structures, (hereinafter “stepped structures”.) Stepped structures can involve steps made of the same or different material layers. The production of these stepped structures demands that lateral etch control be maintained as well as the aforementioned vertical etch control, in order for a stepped structure to be produced. The lateral etch control ensures that a desired lateral recess of one step with respect to the previous step in the stepped structure, is maintained, and the vertical etch control ensures that the height of each step is within specified tolerances.
Systems and methods for control of etch processes of stepped structures have heretofore been deficient. For example, in many prior art etch process flows, measurements of the stepped structure are only taken when the substrate is removed from the etch chamber and taken to a dedicated metrology station for measurement. Information gained from stepped structure measurement is then fed back to the etch process, to adjust for any dimensional deviations in subsequently-processed devices. This results in a large percentage of devices being produced out-of-spec, requiring rework or being discarded, i.e. the device yield is low.
Therefore, there exists a need for advanced control of the etch process of stepped structures. Such an advanced control system and its associated method should preferably have a low cost of implementation, i.e. any hardware modification made to an etch tool to enable the use of such an advanced control method should be of relatively low cost because the hardware modification will need to deployed to a large number of etch tools, as typically exist in any modern semiconductor device fab. The advanced control system will further need to achieve a target accuracy in the etching of stepped structures for the relatively low hardware modification and control method implementation costs. Lastly, the advanced control system should enable a significant increase of device yield.